1. Field of the Invention
The present invention relates to a transfer device of image forming apparatus employing an electro-photographic process such as a laser printer or an electro-photographic copying machine, and particularly a transfer device attaching a roller arranged before transfer area which is hereinafter called “a roller before transfer”, and an image forming apparatus disposing the same.
2. Related Art Statement
An image-forming using an image forming apparatus employing an electro-photographic process such as a laser printer or an electro-photographic copying machine is generally performed as follows.
First, a surface of a photosensitive drum is charged a desired charging voltage Vo such as −630 [V] by a charging device. Next, the surface of the photosensitive drum is exposed by an exposure device such as a semiconductor laser. As a result, an electrostatic latent image is formed on the surface thereof. The surface voltage of the exposed photosensitive drum is attenuated, resulting in that the surface voltage of the latent image forming portion becomes a residual voltage Vr such as −40 [V]. After the electrostatic latent image is formed, a bias voltage such as −460[V], the polarity of which is substantially as same as that of the surface voltage of the photosensitive drum, negative polarity, is applied onto a surface of a developing roller of a developing device and the developing process, the reverse developing process, is performed using charged toner having a negative polarity.
The toner attached to the surface of the photosensitive drum at the developing process is transferred onto a surface of a recording paper by a transfer device which has the recording paper charged a positive polarity, attaching a roller before transfer made of rubber which guides the recording paper to a transfer area. Such a roller before transfer mainly serves as assisting transportation of the recording paper to guide the recording paper to the transfer area, preventing the transferred image from being slipped out by enhancing contact-ability between the surface of the recording paper and the surface of the photosensitive drum and so on.
However, the roller before transfer is arranged in the vicinity of the transfer area, resulting in causing defect that the roller before transfer is easy to be affected by discharging from the transfer device. The surface of the roller before transfer discharged by charge of a polarity, a positive polarity, opposite to the polarity of the charged toner attracts scattered toner or unexpected toner which covers the surface of the photosensitive drum, resulting in that the surface of the roller becomes dirty, thereby the back face of the recording paper becomes dirty.
As a transfer device for preventing the back face of the recording paper from being dirty, there has been known a transfer device provided with a shielding plate made of insulator material which shields discharging action from a transfer wire of the transfer device to a surface of the roller before transfer, disclosed in Japanese Patent laid-open No. 9-197840, and a transfer device provided with a cleaning mechanism which cleans a dirty surface of the roller before transfer.
As to the transfer device provided with the shielding plate as mentioned above, particularly in a case where a photosensitive drum having a small diameter is used, it is natural that the transfer device becomes smaller, so that the width of a discharging opening of the transfer device is partially shielded by the shielding plate, or the width thereof is extraordinarily restricted. The narrower, the width of the discharging opening of the transfer device becomes, the smaller, strength of discharging for the recording paper becomes. Therefore, in order to secure a desired amount of discharging electric charge, higher voltage must be applied to the transfer wire. Thus, there is a problem of safety, e.g., that an amount of generating ozone will be increased and it is not realistic to arrange only the shielding plate in view of cost. Further, a disadvantage as to transferring efficiency will apparently occur because a discharging area in the surface of the photosensitive drum becomes smaller.
On the other hand, as to the transfer device is provided with the cleaning mechanism as mentioned above, particularly in a case where a photosensitive drum having a small diameter is used and the transfer device indispensably becomes smaller, it is difficult to arrange the cleaning mechanism because of small space and further it becomes disadvantageous in view of cost in order to dispose an extra mechanism such as the cleaning mechanism.